The present invention relates to a measuring device for measuring a fuel injection quantity under the conditions corresponding to a real fuel injection.
When fuel is supplied by fuel injection to a diesel engine or a gasoline engine which is provided with an electronic controlled fuel injection system, a measuring device for measuring the fuel injection quantity is required to be developed, planned or examined to minutely control injection fuel quantity. To meet these requirements, various kinds of measuring devices for measuring the fuel injection quantity have been invented. Three examples are hereinafter described.
A piston-type measuring device for measuring the fuel injection quantity has been used in which a cylinder having a piston activated in a given direction is connected to the injection side of a fuel injection valve, thereby detecting a displacement d of the piston and measuring the fuel injection quantity by multiplication of the above-mentioned d and a sectional area of the cylinder S (S.times.d).
A pressure-type measuring device for measuring the fuel injection quantity has been used in which a fuel is injected into a closed vessel (quantity Vo) under a pressure corresponding to a pressure of a combustion chamber and the like, which is called a back pressure, thereby calculating the quantity of injected fuel Vb as follows: EQU dVb/dt=(Vo/E).times.(dP/dT)
An accumulating-type measuring device for measuring the fuel injection quantity has been used in which a fuel quantity injected for a predetermined period of time is detected by multiplication. The fuel quantity for one fuel injection is calculated by dividing the above-mentioned fuel quantity with the injection frequency.
Though the above three measuring apparatuses are well-known, they have some shortcomings, described below, as well as some strong points.
With respect to the piston-type measuring device for measuring the fuel injection quantity in which the injected fuel quantity is calculated by the displacement d and a sectional area of a cylinder S, the sectional area is required to be decreased to improve the resolving power of the measurement. When a minute fuel quantity such as 0.1 mm.sup.3 is to be calculated precisely, however, it becomes very difficult to increase the maximum fuel quantity. Accordingly, it is impossible to measure the fuel quantity precisely within the ability of the fuel injection system, for example, 0-100 mm.sup.3 /stroke.
When a piston is applied, overshoot is generated in measurement due to inertia of the piston so that the precise measurement of the fuel injection quantity is impossible until overshoot ceases. Accordingly, response of the measurement is slow so that the piston-type measuring device is unable to be applied to the measurement of the fuel injection quantity under the conditions that an internal combustion engine is driven at a high speed.
Since a piston is activated by a spring, it is very difficult to maintain the pressure (back pressure) regularly inside a cylinder where the fuel injection is carried out, or to determine the pressure freely. Accordingly, the experiment and measurement are difficult based upon the condition of a real fuel injection.
Furthermore, in the case that the piston is used, there is slide resistance to the piston. The resistance makes it difficult to measure the fuel injection quantity precisely. That is, a seal becomes bad when the resistance is decreased, so that leakage of fuel increases when slide resistance is reduced.
On the other hand, the pressure-type measuring device for measuring the fuel injection quantity calculates fuel injection volume by pressure variation inside a vessel so that accurate measurement is difficult when some air bubbles are mixed. This is because a pressure wave caused by fuel injection is reflected on the air bubble in the vessel and generates a reflected pulse as an error of measurement.
Furthermore, the accumulating-type measuring device for measuring the fuel injection quantity calculates one fuel injection quantity from the fuel quantity that is accumulated within a specified period of time. That makes it difficult to measure the variation of the fuel injection quantity that exerts a large influence to the dynamic characteristic of a fuel injection system, such as the variation of the roughness of the internal combustion engine, for example. Also, according to this measuring device, the accumulation number (stroke number) of the fuel injection quantity is required to be enlarged so as to improve the accuracy of a measurement. Especially, in the case that fuel injection is exercised for a low rotation of the internal combustion engine, it requires a long time to measure and adjust the fuel injection quantity. This problem causes low productivity of a fuel injection system.